Since the creation of unsymmetric cellulose acetate membranes in 1960, there have been rapid developments in the field of porous polymer membranes and the fields of their application have been widening almost unlimitedly, for example in water purification to obtain ultrapure water, drinking water and industrial water, treatment of industrial waste water and municipal sewage, separation and purification of substances in various steps in chemical, pharmaceutical and food industries, and use in medical areas, typically in blood dialysis using artificial kidneys. Such porous polymer membranes mostly have a nonsymmetric structure composed of a skin layer (dense layer) and a support layer (porous layer) and are generally opaque. As regards the use of porous polymer membranes in areas where transparency is required, Japanese Patent Application Laid-open No. 48026/1989 (JP-A-64-48026), for instance, proposes liquid crystal panels comprising a transparent porous film impregnated with liquid crystal molecules and sandwiched between two panels. This reference, however, gives no mention of the material of the transparent porous film.
In Japanese Patent Application Laid-open No. 159791/1991 (JP-A-3-159791), there is proposed a thermal transfer recording sheet in which a transparent porous resin layer is used. While this reference mentions an ethylene-vinyl chloride copolymer as the resin for forming the porous resin layer, it has no mention of the process for forming the porous resin layer.
In Japanese Patent Application Laid-open No. 96332/1991 (JP-A-3-96332), there is proposed a superficially porous film which is a laminate film having a porous surface layer with the peak pore size in the pore size distribution curve being 0.06 to 2.0 .mu.m and with less than five undulations per 4 .mu.m which are higher than 0.2 .mu.m, and having a haze of not more than 10%. The porous layer of this film is formed by applying a coating composition comprising a water-dispersible polymer and colloidal silica, followed by drying. Therefore, its water resistance is low, and the transparency is also low due to the difference in refractive index between the water-dispersible polymer and colloidal silica.
In Japanese Patent Application Laid-open No. 145005/1991 (JP-A-3-145005), a thin electrolyte membrane is proposed which comprises a porous solid polymer membrane having independent through holes with a mean diameter of 0.01 to 0.7 .mu.m, with the holes being filled with an ionic conductor material. This porous solid polymer membrane is produced by irradiating a thin polymer membrane with charged particles and etching the tracks of the charged particles with an alkali to form pores. The porous membrane obtained by this method is semitransparent. For rendering it transparent, it is necessary to fill the pores with a liquid having a refractive index approximately equal to that of the membrane material.
In Japanese Patent Application Laid-open No. 263982/1992 (JP-A-4-263982), a recording sheet is proposed which comprises a substrate provided with a porous layer of cellulose acetate and a colorant-carrying layer consisting of pseudoboehmite as the top layer. The porous layer is formed by dissolving cellulose acetate in a solvent such as acetone, adding magnesium perchlorate or the like in a suitable amount and applying the resulting coating composition, followed by drying. The transparency of the film is reduced by precipitation of magnesium perchlorate or the like upon evaporation of the solvent.
In Japanese Patent Application Laid-open No. 86251/1986 (JP-A-61-86251), a recording sheet is disclosed which is a laminate comprising a substrate film layer, a liquid-absorbing layer formed on at least one side of the substrate film layer, and a porous, thin plastic membrane layer (a polyethylene or polypropylene film with a large number of pores having a diameter of 0.01 to 0.1 .mu.m formed therein) bonded to the liquid-absorbing layer by means of a hot press. However, no description is given of the method of preparing the porous, thin membrane layer.
On the other hand, the following methods, among others, are known for the production of porous polymer membranes: (1) the phase separation method which comprises subjecting a polymer to micro phase separation using a good solvent and a poor solvent; (2) the foaming method which comprises forming a polymer to form pores; (3) the stretching method which comprises stretching a polymer film; (4) the irradiation method which comprises exposing a polymer film to radiation to form a hole; (5) the extraction method which comprises preparing a film from a polymer or inorganic salt soluble in a solvent and a polymer insoluble in the solvent and removing the soluble component by extraction with the solvent to cause hole formation; and (6) the sintering method which comprises partially fusing polymer particles or partially binding polymer particles using a binder and utilizing the resulting voids or gaps among the particles as pores.
Among the above methods, the phase separation method is the commonest. Thus, for example, Japanese Patent Application Laid-open No. 246095/1991 (JP-A-3-246095) discloses a technique for forming a porous plastic surface layer by applying a plastic solution to a transparent plastic film substrate, followed by wet coagulation. According to this reference, a solution of a saturated copolyester resin in dimethylformamide is applied to a substrate and coagulated in water and the coated substrate is further immersed in a hot water and dried, whereby a white, devitrified, porous plastic surface layer is formed.
However, the phase separation method generally gives nonsymmetric membranes comprising a dense surface layer and a porous underlying layer(void layer). The void layer has a large pore size, opacifying the porous membranes. If transparent membranes are formed, they will be nonporous membranes unusable in the fields where pores are useful. It is thus difficult to obtain membranes or films having a porous structure while retaining high transparency.
Accordingly, it is an object of the present invention to provide highly transparent porous membranes and a process for producing the same.
Another object of the invention is to provide porous membranes high in transparency and ink absorption and excellent in water resistance, and a process for producing the same.
A further object of the invention is to provide a process for producing highly transparent porous membranes with high productivity.